Get anti-aliased depth map in cycles render
You can easily extend the mist pass again.


  When measuring pixel values with &#9096; Ctrl LMB, be sure to set the display color color management to None otherwise your values will not be read linear.
  



Define an infinity limit, a value which is the maximum distance from the camera. Infinity will assume this value as well, objects beyond will clamp to this value. I used 1000. Set the mist depth to this value.
Set the mist start to 0.


Render a separate mist pass.

In Compositing:


Invert the Mist pass (1 - Mist).
Multiply it with the depth limit.




Remember to save as an .exr to retain values > 1.